Eu uso a conexão SSL para conectar o cliente da web ao servidor. Funciona sem problemas por um longo tempo. Mas a partir de ontem, dá o seguinte erro, alguém pode me dizer o motivo.

javax.net.ssl.SSLException: Connection has been shutdown: javax.net.ssl.SSLHandshakeException: Received fatal alert: certificate_unknown
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.checkEOF(SSLSocketImpl.java:1172)
        at com.sun.net.ssl.internal.ssl.AppInputStream.read(AppInputStream.java:65)
        at net.schubart.fixme.internal.MessageInput.readExactly(MessageInput.java:166)
        at net.schubart.fixme.internal.MessageInput.readMessage(MessageInput.java:78)
        at cc.aot.itsWeb.ClientWriterThread.run(ClientWriterThread.java:241)
        at java.lang.Thread.run(Thread.java:619)
clientWriter.ready
Caused by: javax.net.ssl.SSLHandshakeException: Received fatal alert: certificate_unknown
        at com.sun.net.ssl.internal.ssl.Alerts.getSSLException(Alerts.java:174)
        at com.sun.net.ssl.internal.ssl.Alerts.getSSLException(Alerts.java:136)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.recvAlert(SSLSocketImpl.java:1586)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:865)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1029)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.writeRecord(SSLSocketImpl.java:621)
        at com.sun.net.ssl.internal.ssl.AppOutputStream.write(AppOutputStream.java:59)
        at java.io.OutputStream.write(OutputStream.java:58)
        at net.schubart.fixme.internal.Message.write(Message.java:267)
        at net.schubart.fixme.internal.MessageOutput.writeMessage(MessageOutput.java:53)